The present invention relates to an input circuit for an output stage which is inserted between a signal line and an output stage controlled by the signal in order to suppress spurious pulses which may be generated, for example, by electromagnetic induction phenomena in the signal line.
When output stages are controlled via an electric line having a non-negligible length, the problem always arises of distinguishing between xe2x80x9cintentional controlxe2x80x9d and interference in the form of electromagnetic induction phenomena on the line, for example. In the first case, the output stage should receive a current from the line; in the second case it should preferably ignore the interference signal erroneously applied. This problem is known as xe2x80x9celectromagnetic compatibilityxe2x80x9d (EMC). The desired electromagnetic compatibility can be achieved in principle by evaluating the different voltage levels applied to the output stage; intentional control signals usually have a fixed voltage range, which is above the limit value characterizing a given application. In contrast, signals caused by electromagnetic interference have a smaller voltage range which can be distinguished from one interference event to the other and is usually lower than the intended control signals. Thus, for example, in the case of an ignition output stage of a motor vehicle, xe2x80x9cintendedxe2x80x9d control takes place with voltages higher than 3 V, whereas signals caused by electrical or electromagnetic interference on the control line of such an ignition output stage have typical values of up to 2 V. Although these voltages, which are lower than the setpoint value of 3 V, do not entirely activate the ignitor, the interference signal levels may be sufficient, especially at high temperatures, for allowing the flow of load currents greater than 0.1 A, so that an effect of the interference can be detected in the load circuit.
Various discrete input wirings of such an output stage are known for suppressing the interference signals; however they are expensive to manufacture due to their discrete design and require considerable space.
Comparators integrated on a single semiconductor substrate are also known which generate a discrete output signal whenever the level of an input signal applied exceeds a reference value. Such comparators may be connected upstream from an output stage in order to suppress all incoming pulses that do not exceed the reference value (in the above case of an ignition output stage, a reference voltage of 3 V, for example). These comparators, however, require a supply voltage and an externally supplied reference voltage in order to perform their function. Supplying this reference voltage from a certain distance is inconvenient because in this case the reference voltage supply lead is subject to interference in the same way as the control line. In contrast, if the reference voltage is generated directly in the control circuit, the complexity, manufacturing costs, and required space are increased again.
The control circuit for an output stage having the features specified in claim 1 offers the advantage compared to the related art in that it makes reliable suppression of interference signals possible without the need to provide a reference voltage; it operates without a supply voltage, and for its operation only requires components that are easy to integrate on a semiconductor substrate.
As long as no signal is applied to the signal input of the control circuit, both switch stages are in their first state. If the potential at the signal input increases due to an incoming signal, the potentials of both output terminals follow suit, so that initially a rising potential is also output by the signal output. With increasing potential at the signal input, the threshold value of the second switch stage is exceeded first, which switches over to its second state in which its output terminal is drawn to ground. Since this is also the signal output, no more signals are output; the pulse received at the signal input is suppressed. If the potential applied to the signal input continues to rise, the threshold value of the first switch stage is also exceeded, its output is drawn to ground, whereupon the second switch stage returns to its first state in which the potential at the output terminal becomes equal to that at the signal input. Thus an input signal which exceeds the threshold value of the first switch stage is output at the signal output, while lower-intensity signals are suppressed.
Each switch stage can be formed in a simple manner by a resistor and a transistor, the resistor being connected between the collector of the transistor and the signal input, and the emitter of the transistor being connected to ground.
The base of this first and/or second transistor can directly form the control terminal of the first and/or second switch stage, respectively; however, in the case of the first switch stage, a third transistor is preferably provided whose base forms the control terminal of the switch stage and whose emitter controls the base of the first transistor. Thus, a steeper increase in the potential at the base of the first transistor and thus sharper separation between input signals greater and smaller than the threshold value of the first switch stage are achieved.
A fourth transistor is preferably also provided as part of the second switch stage; the collector of this transistor is connected to the signal input, its emitter forms the output terminal of the second switch stage, and its base is controlled by the collector of the second transistor. This transistor is conductive between emitter and collector in the first state of the second switch stage, and in its second state it blocks the connection between the signal input and the signal output of the control circuit.
The emitter of the fourth transistor can be connected to ground via a resistor.
The second threshold voltage of the control circuit is the voltage above which small interference signals are effectively suppressed. Expediently, it should be as low as possible, preferably in the range of the semiconductor boundary layer junction voltage. The first threshold voltage can be selected essentially freely depending on the intended application of the control circuit; in the case of an application in conjunction with an ignition output stage of a motor vehicle, the first threshold voltage is preferably higher than 2 V.